Answer:
Because Jefferson was actually against slavery. Even while he owned slaves, it sickened him. however Jefferson as in some of his writings, feared that unless given a direction Slaves would not be productive members of society as they would not, and that due to their lack of education would rebel against their previous owners even thouh they became free.
Jefrerson battles with the idea of freeing slaves and the idea of the consequences of freeing the slaves giving them nothing and having come from nothing. to him it would have been just as bad to free them with nothing to stand on as opposed to keeping them in their current state until something better could be arranged for them.